The present invention relates to extruders of a type capable of receiving scrap plastic at ambient temperatures and discharging it as a melted extrudate. The scrap supplied to such machines may be of a very high bulk type, such as, comminuted polyethylene or polyvinyl chloride film. There is a great advantage, particularly when subjecting heat-sensitive high-bulk materials to a melting process, to reduce the bulkiness to the fullest extent possible before the application of heat. In this manner, the heat transfer may be maximized without the hazard of overheating and the extruder may be efficiently utilized by achieving a satisfactory through-put rate. In any event, the bulkiness must somehow be reduced in compression ratios that may range up to 25 to 1 to achieve practical through-put of the work material.
Among the prior art devices for reducing bulkiness are the motorized hopper screw feeders which fail to achieve feed stock compression ratios greater than about 4 to 1 and, moreover, are quite expensive.
Another expedient of the prior art is to provide a large feed opening, e.g., the so-called, "wedge key" feed throats, wherein the capability of the extruder screw to pull in a large volume of work material is obtained primarily through a reduction of the diameter of the screw core in that section of the screw within the feed throat. The feed throat is defined as that section of the barrel providing a lateral feed port plus about one turn of the screw and the surrounding barrel section downstream of the feed port. The capability of the "wedge key" feed throat, however, is limited by the strength of the screw in the feed section and the inability to obtain high compression ratios.
It is known also to construct extruders wherein the screw and the barrel have a large diameter within the feed section than in the melt section. An important disadvantage with this machine is that the cleaning of the extruder screw requires dismantling or removal thereby causing the cleanup operation to be quite time consuming.